The Doctor, Ian & Barbara explore the jungle on the planet Mechanus examining the plants which are surprisingly mobile. Ian thinks it's the perfect place to fight the Daleks. Vicki sneaks out of her hiding place in the Dalek's time machine. The Doctor and his group follow some lights to a cave where Vicki finds what looks like a weapon. Vicki hammers on the Tardis door but nobody is there to let her in while the plants continue to attack her. Vicki's screams are heard by the others so Ian & The Doctor leave to find her. The Daleks engage the attacking plant life. Meanwhile the robot Doctor finds Barbara and tells her Ian is dead. The real Doctor and Ian, carrying the unconscious Vicki return to the cave to discover Barbara gone. Ian searches for her but when Vicki wakes she thinks the Doctor is the robot but when Ian returns she is convinced and tells them of the robot. Ian finds Barbara who is attacked by the robot. The robot flees the scene, returning when the real Doctor arrives. There's an argument but they figure out which Doctor is the robot when he gets Vicki's name wrong and calls her Susan. The real Doctor defeats the robot. The Daleks guarding the Tardis decide to search the jungle at daybreak. The travellers spend the night in a cave during which they are monitored by a mechanical eye. In the morning, with clearer skies, the travellers are able to see a city raised high above the jungle they are in. The Daleks approach them, but the Doctor pretends to be the robot duplicate - oh look there's the broken neck ring on a normal coloured Dalek, I assume the Supreme has had a respray. Good grief, five weeks in and two of the Dalek power slats have been damaged already! The Doctor attempts to deceive them but fails and they attack. A door opens at the back of the cave revealing a strange, almost spherical Mechanoid which invites them to enter.....

Yeah this episode struggles. It's main problem is that for most of the time it's obvious which is the Doctor and which is the double due to a different actor playing the duplicate in most of the scenes. But I think lessons are learnt here and the next time there's a duplicate of the doctor the two are kept apart allowing Hartnell to play both roles. As well as the Time Travel, the duplication of another being shows up as a Dalek ploy in a later story too: Many year later the Daleks use this technique in The Resurrection of the Daleks.